Early Mdina amethyst is a slightly brownish, more aubergine-y shade, often the brownishness is made more so by the patches of yellow which arise from the use of silver salts in the clear casings, the yellow plus the amethyst can sometimes go quite brown.
There is a post-Harris purple which is much brighter (more akin to wfs aubergine - as in the strapped Baxter pieces). This is what Mark Hill refers to as the colours becoming more "jewel-like" in his book, "Michael Harris: Mdina Glass & Isle of Wight Studio Glass".
There is another recent amethyst, which is much more of a deep purple :sm: - slightly more blue-ish. It's used in a densely splodged manner to create the appearance of black, (notably in the "Black and Silver" range). I've not seen it used in any other way.
This is a nice little strapped pot with the early, brownish amethyst colour used - does the colour have patches of different shades in it? It should have a stopper, but they're often missing, and it doesn't spoil the piece not to have it. The stoppers are clear, decorated internally with swirls of amethyst glass and often streaks and blobs of deep yellows and ochres from the silver salts reacting with the clear glass.
(I have to confess, I nicked the stopper from my one of these and put it in a bottle which benefits from it.)
When these and many other bottles were sold, I believe stoppers were maybe optional, possibly at extra cost, and they were made to fit most bottles - there is not a fit between the stem and the neck. I can't remember where I heard that and I have no evidence to back it up.
Basically, a stopper is desirable.
But unless there is a ground bit inside the neck of something, it is not neccessarily missing it's stopper.